Tire dealers and service shop operators should understand a few facts in order to manage battery drain diagnosis effectively and profitably.
In this column and the next one, I'll give you a super-simplified explanation of why abnormal “drains” kill batteries. Then I'll outline several steps that simplify both the diagnosis and charging for the work.
For openers, current is electrical volume; current is measured in amperes (amps). Suppose you turn on an electrical device in your car such as the radio or dome light. If so, then amps flow out of the battery, through the radio or dome light and then back to the battery. This loop from the battery to the device and back to the battery is commonly called a complete circuit.
To shut off the radio or dome light, you turn off a switch of some kind. When you turn off a device, you're actually opening a switch. Opening a switch “opens up” or “breaks open” the circuit so the current stops flowing.
Suppose you're running late. You reach the conference center, turn on the dome light for a moment and locate your wallet. Then you turn off the dome light and race into your meeting. However, the contacts inside the dome light switch happen to stick together. (In technician speak, this switch is “shorted together.”) You overlook that the dome light is still on and amps continue flowing through the dome light circuit until your car's battery dies.
Technicians often call the current flowing through the shorted dome light circuit a “short” or a “drain.” A good technician will recharge the battery and then inspect the car for obvious faults such as a dome light stuck on, a glove box light that's hot, etc.
The shorted dome light switch causes an abnormal battery drain. However, many normal battery drains occur on modern vehicles. That is, current constantly flows through various electronic components after the driver shuts off the ignition switch.
Normally, this current flow is miniscule—perhaps 50-100 milliamps or so. (A milliamp is one-thousandth of an ampere.) This constant flow of current maintains essential memory in a wide range of components: radio, engine control computer, transmission control computer, air bag control computer, et al.
The number of on-board devices that consume current when the ignition switch is off has bloomed over the last 20 years. However, this constant but small current flow is OK provided the vehicle is healthy and the battery has the proper electrical capacity.
To recap briefly, current continues flowing through a variety of on-board electrical components even though the ignition switch is off and the component itself appears to be shut off. Technicians call this unavoidable amount of current flow key-off drain, key-off electrical load, key-off parasitic drain or key-off parasitic load.
Rest or sleep mode is the next aspect of this topic. You see, the current flowing through these on-board components is considerably greater when the engine's running than when the ignition switch and engine are shut off.
Eventually, these “memory-hungry” components shift into a relaxed state known as sleep mode or rest mode.
During this sleep state or rest mode, the current flow through the component drops into that miniscule (milliamp) range I described earlier.
After the driver shuts off the ignition switch, it takes a certain amount of time for each and every memory-hungry component to shift into rest mode. The time required to trigger rest mode varies widely from component to component and vehicle to vehicle. In some cases, the wait time is as little as 20-30 minutes. But as the vehicles and systems have become more sophisticated, rest mode for some devices may not occur for 60 to 120 minutes—sometimes even more.
Unfortunately, many vehicle man¬ufacturers do not publish official values for normal key-off current drain or for the time required to shift into sleep mode.
Be sure to tune in to my next column for practical guidelines on battery drain diagnosis.